1. Field of the Invention
This invention concerns injection molding apparatus and methods and more particularly manifold supplied multicavity molds.
2. Description of the Prior Art
Injection molding is a process in which molten plastic material, pressurized by an injection machine, is directed into a mold cavity through a mold nozzle. Molds are formed with openings called gates through which the material passes in entering the mold cavity. The flow of material is controlled in so-called "open loop" systems by cycling of the injection molding machine, i.e., each cycle of the injection machine causes a predetermined volume of material to flow into the mold cavities.
Multicavity molds are sometimes used to gain greater efficiency, in which the output of the injection machine passes through runners in a manifold to each of a plurality of mold nozzles each associated with a respective mold cavity.
Difficulties are encountered in open loop systems in obtaining exactly the correct flow to each cavity In practice, balancing of the flow is obtained by "fine tuning" the gate orifices by machining the diameters to obtain the correct flow.
Another approach, as disclosed in U.S. Pat. No. 4,279,582, is to use hydraulically actuated shut off valves associated with the gates to time the duration of flow to in turn enable precise control over the volume of material flow to each mold cavity.
This first mentioned approach is tedious and time consuming and does not allow for reducing the flow in the event too much flow is inadvertently created by machining too large a gate opening.
In the second mentioned approach, greatly increased costs and complexity are associated with the incorporation of hydraulic actuators and controls needed for precisely timed gate valving means. Furthermore, the complete closing of the gate depressurizes the material in the mold cavity during curing, and shrinkage cannot be made up by additional material.
In U.S. Pat. No. 3,561,062 there is disclosed a complex sleeve valve and mating port arrangement for adjusting the flow to a downstream injection port, but this approach requires complex additional structure, and controls the flow a substantial distance from the point of injection at the gate.
If an adjustable position restriction were placed at the gate, bleeding would likely occur, since a needle could not be placed there, nor could a diverging plug cavity for absolute flow nozzles.
Furthermore, such a restriction would not allow use of the various full body, bodyless, and absolute flow nozzles.
Therefore, it is the object of the present invention to provide an apparatus and method for balancing material flows in manifold supplied multicavity molds which does not necessitate machining of the mold parts or the use of elaborate auxiliary shut off or adjustment valving systems.
It is a further object to provide such apparatus and method for balancing flows usable with standard full body, bodyless, and absolute flow nozzle configurations, which maintains pressurization of the mold cavities during curing, while insuring proper separation of the cured and uncured material.